Update: After having a good heart to heart with my boss he agreed that my job was undervalued in terms of pay (not me the job) and has agreed to a substantial pay increase

I feel vindicated now

I am still paid less but feel satisfied with that a) because the other PA has been here longer, but mainly because b) the job was evaluated properly and that is ultimately what I wanted.
Happy ending and thanks everyone for the advice

Callie.